Colorectal cancer (CRC) is the fourth most common cancer and the second leading cause of cancer deaths in the United States. Lower endoscopic examinations including flexible sigmoidoscopy and colonoscopy have been demonstrated to reduce the risks of incident CRC and CRC-specific mortality through detection and removal of precancerous lesions and early detection of CRC. However, lower endoscopy utilization is still less than 60% among persons aged 50 years or older. Previous research identified significant geographic variations in CRC screening, incidence, and survival. Small-area socioeconomic deprivation has been associated with CRC-related outcomes, but was unable to explain the observed geographic disparity in CRC-related outcomes, suggesting that other community-level characteristics contribute to geographic disparities in CRC-related outcomes. Access to healthcare is an important prerequisite to obtaining quality care and thus a determinant of health outcomes. Low spatial accessibility to mammography service has been associated with an increased risk of late-stage breast cancer. Little is known whether spatial accessibility to lower endoscopy services contributes to the geographic variation in CRC-related outcomes. Therefore, we will explore the hypothesis that geographic disparities in CRC screening, incidence and survival are attributable to spatial accessibility to lower endoscopy services. Two specific aims will be addressed: (1) Compute and map small-area spatial accessibility to lower endoscopy services across the United States including 50 states and Washington DC, using a GIS, Medicare data, and spatial statistical modeling, and (2) Examine the effects of small-area spatial accessibility to lower endoscopy services on CRC-related outcomes, including the risks of incident CRC, advanced stage at diagnosis, and CRC-specific mortality. We will use three different spatial approaches to quantify and map small-area spatial accessibility to lower endoscopy services based on the locations of physicians who performed a sigmoidoscopy or colonoscopy using Medicare data. Then, we will link the small-area spatial accessibility measures with data from the high- quality NIH-AARP Diet and Health Study including 566,407 persons to prospectively assess the effects of spatial accessibility to lower endoscopy services on CRC incidence, advanced stage at diagnosis and CRC- specific mortality. This is the first study to examine the geographic disparity in spatial accessibility to lower endoscopy services and prospectively evaluate its effect on CRC outcomes. The results of the proposed study will provide evidence for the development and implementation of evidence-based multilevel interventions to increase lower endoscopy use and thus improve CRC-related outcomes. In addition, the proposed study will develop a spatial accessibility map which will help local and state policy makers improve the allocation of lower endoscopy services.